Modified fluidized bed

ABSTRACT

A process (and apparatus therefor) for carrying out thermally initiated chemical reactions or physical changes by heating a particular substrate in a fluidised bed having hotter zone(s) and cooler zone(s) between which the fluidised particles circulate, the dwell time in the hotter zone(s) being sufficiently short so as to prevent the particles undergoing an undesirable thermal transformation, and/or the dwell time in the cooler zone(s) being sufficiently long to allow chemical reaction and/or physical changes to take place which prevent undesirable thermal transformations occurring in the hotter zone(s). The fluidised bed process is applicable, for example, to the polymerisation of a low molecular weight poly(ethylene terephthalate) and to a process for the preparation of poly (ethylene terephthalate) from a terephthalic acid and ethylene oxide.

This invention relates to a process for causing a material to undergo achemical reaction or physical change in a fluidised bed, especially whenat least one component of the fluidised bed is susceptible to anundesirable thermal transformation.

The type of chemical reactions or physical changes to which thisinvention is directed are those in which at least one of the reactantsor components of the material is in a solid particulate state and thedesired chemical reaction or physical change is initiated and/ormaintained thermally. It will be appreciated that it is desirable toraise the solid particles to a temperature at which the reaction orphysical change proceeds at an acceptable rate, but to avoid theabove-mentioned undesirable thermal transformation. However, suchtransformations, for example, degradation, carbonisation, over-reactionor melting, commonly result from a combination of temperature and time.For example, whereas a material may be able to withstand a certainelevated temperature for a short time, it will undergo the undesirablethermal transformation if it is maintained at that temperature for alonger time. Furthermore, if the undesirable thermal transformationinvolves melting of the solid particles, this can cause them to sticktogether, causing so-called "rat-holes" to appear in the bed, and, ifagglomeration becomes sufficiently severe, to inhibit fluidisationaltogether.

We have now devised a way of conducting chemical reactions or physicalchanges in a fluidised bed, which minimises or avoids theabove-mentioned difficulties.

According to one aspect of our invention, we provide a process forcausing a material to undergo one or more thermally-initiated chemicalreactions or one or more physical changes in which the material or atleast a component thereof is in a solid particulate state and ismaintained as a fluidised bed, the fluidising gas being supplied in twoor more zones, comprising at least one cooler zone in which thetemperature of the gas is below a temperature at which said particulatematerial is susceptible to an undesirable thermal transformation and atleast one hotter zone in which the temperature of the gas is above thetemperature at which said particulate material is susceptible to theundesirable thermal transformation, the zones being arranged so that theparticles of the bed are constantly moving between them, the dwell timein the hotter zone or zones being sufficiently short so that theundesirable transformation does not take place before the particles moveinto the cooler zone or zones and/or the dwell time in the cooler zoneor zones being sufficiently long to allow chemical reaction(s) orphysical change(s) to take place so that when the particles move intothe hotter zone or zones the undesirable transformation does not takeplace, the temperature of the zones being selected so as to cause thechemical reaction(s) or physical change(s) to take place.

The process may be controlled, for example, such that the dwell time inthe hotter zone(s) is sufficiently short so that the undesirable thermaltransformation does not take place before the particles move into thecooler zone(s), the temperature of at least the hotter zone(s) beingsufficiently high to cause chemical reaction(s) or physical change(s) totake place. This mode of operation is especially applicable to processesinvolving a single thermally-initiated chemical reaction system.Preferably the cooler fluidising zone(s) is arranged to surround thehotter fluidising zone(s) so that at least some of the particles areborne upwards in the hotter gas and, on reaching the top of the bed,spill over into the surrounding gas where they travel to the bottom ofthe bed and are later re-entrained in the hotter gas.

However, it will be appreciated that at least some of the particles maybe cycled between the zones in other directions, for example, transverseto the bed, if other modes of circulation are superimposed on thefluidised bed circulation, for example by the use of stirred fluidisedbeds.

Thus, in one embodiment the bed may comprise either a single hotter zoneat the centre of the bed or a plurality of such zones spaced about thearea of the bed. However, the hotter zones must be sufficiently wellseparated to provide a comparatively large reservoir of cooler gasaround each of them, otherwise the whole bed would quickly attain thetemperature of the hotter gas and nullify the desired effect.

Generally, the cross-sectional area of the hotter zone(s) will besmaller than that of the surrounding cooler zone(s) so that the hotterparticles are concentrated in a narrower zone or zones. Alternatively,the process may be controlled such that the dwell time in the coolerzone(s) is sufficiently long to allow chemical reaction(s) or physicalchange(s) to take place so that when the particles move into the hotterzone(s), the undesirable transformation does not take place. This modeof operation is especially applicable to processes comprising one ormore reactions occurring at a lower temperature and one or morereactions occurring at a higher temperature, and wherein the meltingpoint of the solid particles comprising the bed is low or lowered at thestart of the reaction sequence and increase as the reaction sequenceproceeds.

Preferably the hotter fluidising zone(s) is arranged to surround thecooler fluidising zone(s) so that at least some of the particles areborne upwards in the cooler gas and, on reaching the top of the bedspill over into the hotter surrounding gas, where they travel to thebottom of the bed and are later re-entrained in the cooler gas. It willbe appreciated that at least some of the particles may be cycled betweenthe zones in other directions, for example transverse to the bed, if,for example, the bed is stirred.

Thus, in another embodiment the bed may comprise either a single coolerzone at the centre of the bed or a plurality of such zones spaced aboutthe area of the bed. However, the cooler zones must be sufficiently wellseparated to provide a comparatively large reservoir of hotter gasaround each of them, otherwise the whole bed would quickly attain thetemperature of the cooler gas and nullify the desired effect.

The dwell time of the particles in the hotter zone or zones is criticalif the reaction temperature is to be kept high enough to promote fastreaction, but the undesirable thermal transformation is to be avoided,for example in processes involving a single thermally-initiated chemicalreaction system. Thus the depth of the bed must be regulated so that itis not so deep that the dwell time is unduly long nor so shallow thatthe gas "channels" through the bed.

When two or more chemical reaction systems occur at differenttemperatures, and the melting point of the solid particles is low orlowered at the start of the reaction sequence and increases as reactionproceeds, it is important that the dwell times in the hotter and coolerzones (and hence the volumes required for the hotter and cooler zones)are such as to achieve the desired extent of reaction whilst avoidingundesirable thermal transformation. Thus, the dwell time in the coolerzone(s) may be regulated so that it is sufficiently long to ensure thatany initially formed molten or partially molten products undergo furtherchemical reaction to form solid particulate products having a highermelting point before moving into the hotter zone(s). At the same time,the dwell time in the hotter zone(s) may be regulated to ensure adequatefurther reaction. It will be appreciated that as the reaction proceedsto give products of increased melting point, the temperature of thecooler zone(s) may also be increased.

Apart from the difference in temperature of the gas supplies, theirrelative superficial velocities and flow rates must be chosen so thatthe particles are raised to the required temperature in the hotter gas,but either not kept at that temperature too long and/or the temperatureis not raised too quickly. However, the velocities must always be abovethat required for incipient fluidisation, but the mean velocity over thebed must not, of course, exceed the free falling velocity of theparticles, to avoid pneumatic transport of the particles out of the bed.

It will be appreciated that considerable variations in the relativerates of the gas flow, linear velocity and bed depth may be used toachieve our desired effect. However, the optimum values may be simplydetermined by routine experimentation using procedures well-known in theart, for example as described by Davidson and Harrison in "FluidisedParticles" published by Cambridge University Press 1963.

The solid particulate material is most conveniently heated bycontrolling the temperature of the gas streams to the hotter and coolerzones. This is preferably achieved by providing separate gas supplieswhich are heated independently, for example, by passage throughheat-exchangers. If a chemical reactant is included in the gaseousphase, it may be incorporated in the gas supply to either zone.

It is generally convenient to carry out the process of our inventionbatchwise, continuing the process until all the particles have undergonethe desired chemical or physical change by virtue of recirculationbetween cooler and hotter zones. However, it is possible to run theprocess on a continuous basis, for example, using a multi-stage reactor.

According to a further aspect of our invention we provide a fluidisedbed apparatus comprising a vessel having a gas-permeable baseplateprovided with means whereby a gas may be caused to pass therethrough, atleast one nozzle or jet located in the vessel above the baseplate andprovided with independent gas-supplying means, each gas-supplying meansbeing associated with a heat-exchanger, whereby each gas supply may beheated or cooled independently to a pre-determined temperature.

The gas permeable baseplate may conveniently comprise a sintered metalor glass disc or a metal plate having a multiplicity of small holes, thefluidising gas or gases being supplied over substantially its wholearea, e.g. by way of a plenum chamber.

The apparatus may comprise a single vertical jet passing through thecentre of the baseplate or there may be several vertical jets spacedabout the baseplate. However, when there are several jets they must besufficiently well-spaced to define separate hotter and cooler zones.

Alternatively the jet or jets may comprise at least one disperser havinga plurality of side ports or like means adapted to produce initiallysubstantially horizontal flows of gas within the vessel. The disperseror dispersers may be used in combination with a stirrer or stirrersadapted to rotate within the vessel. The apparatus may suitably comprisea single such disperser passing through the centre of the baseplate orseveral such dispersers spaced about the baseplates which aresufficiently well spaced to define separate hotter and cooler zones.

The jet or jets may also comprise at least one hollow shafted stirrerlocated above the baseplate, each stirrer being provided with aplurality of holes adapted to produce initially substantially horizontaland rotating flows of gas. Preferably the holes are located in thestirrer shaft. The apparatus may suitably comprise a single such stirrerlocated above the centre of the base-plate or several such stirrersspaced about the baseplate which are sufficiently spaced to defineseparate hotter and cooler zones.

The use of jets in the form of the aforesaid dispersers or hollowshafted stirrers provided with a plurality of holes, has the effect ofwidening the zone or zones associated with said jets, which isespecially useful for increasing the dwell time of particles in a coolerzone or zones surrounded by a hotter zone or zones. The use of stirrers,for example in combination with dispersers, or as a hollow shaftedstirrer provided with a plurality of holes, is particularly advantageouswhen handling solid particulate materials which become strongly cohesivewhilst undergoing chemical change(s) or physical change(s) in the bed.

It will be appreciated that a variety of chemical reactions or physicalchanges may be carried out using our process. For example, a solidmaterial may be heated in an inert gas to a temperature at which adesired chemical reaction occurs; or a solid material may be reactedwith a gaseous reactant, which may either be used as, or as a componentof, one or both of the gas streams. Alternatively, atemperature-sensitive material may be dried by subjecting it to aninert, heated gas stream.

One particularly suitable process which may be carried out using theprocess of our invention is the polymerisation of a low molecular weightpoly(ethylene terephthalate), that is, raising its molecular weight (ascharacterised by its intrinsic viscosity, IV) by heating. Poly(ethyleneterephthalate) is commonly prepared by reacting high purity terephthalicacid with ethylene glycol in the melt phase at approximately 285° C.Essentially, the reaction proceeds in two stages, the first involvingesterification, to produce a mixture of mono- and bis- hydroxyethylterephthalate and the second involving polycondensation andpolyesterification to yield low molecular weight poly(ethyleneterephthalate). This last-mentioned product is commonly referred to as a"pre-polymer". This pre-polymer must be further polymerised to raise itsIV to the range 0.67 to 1.0 to give a commercially useful polymer. Thisis achieved by heating the pre-polymer commonly at a temperature of atleast 235° C. in the presence of a catalyst.

Since ethylene glycol and water are evolved during this polymerisationprocess, it is preferred that the solid pre-polymer is in particulateform, to increase its surface area and hence facilitate removal of theethylene glycol and water. The process may be carried out by heating theprepolymer particles in a stirred vessel; but there is a clear potentialadvantage in making the particles the solid phase of a fluid bed. Theparticles could then be heated by means of a heated fluidising gas whichwould not only cause the polymerisation reaction to take place, but alsoaid removal of the evolved ethylene glycol or water. However, if asimple fluidised bed is used, that is, one in which the whole of thefluidising gas is raised to a temperature at which the reaction takesplace at an acceptable rate, the particles of pre-polymer become"sticky", due to surface melting, and agglomerate sufficiently toproduce "rateholes" in the bed or even to such an extent thatfluidisation is no longer possible.

In contrast to this, we have found that the polymerisation reaction maybe carried out in a modified fluidised bed according to our inventionwith greatly reduced, or even no, agglomeration of the particles andwherein the fluidising gas in the cooler zone(s) is preferablymaintained between 205° and 210° C. and the gas in the hotter zone(s) orjet(s) is preferably maintained between 210° and 265° C., for examplebetween 230° and 250° C. In this process an inert gas is used, since itis only required as a heating and fluidising medium. Any inert gas maybe used, but nitrogen is generally most convenient.

Another particularly suitable process which may be carried out using theprocess of our invention is the reaction of solid terephthalic acid withgaseous ethylene oxide to give solid poly(ethylene terephthalate). Sucha process is described, for example, in British Patent Specification No.1,387,335 (Imperial Chemical Industries). The process comprises aninitial esterification stage in which a mixture of solid mono- and bis-hydroxyethyl terephthalate is formed and a second polycondensation orpolyesterification stage (with more terephthalic acid) to givepoly(ethylene terephthalate). The esterification stage may be carriedout in the presence of suitable catalysts, for example organic basessuch as tertiary amines, tertiary phosphines, quaternary ammoniumhydroxides, and quaternary phosphonium hydroxides. An esterificationcatalyst may be used which is volatile under the conditions of thereaction and is conveniently introduced into the bed with an inertfluidising gas. When a non-volatile catalyst is used, it is convenientlyintroduced by incorporating with the solid terephthalic acid. The secondstage may be catalysed by polycondensation catalysts, for examplecompounds of antimony, germanium, tin or titanium, or bypolyesterification catalysts such as titanium compounds.

The process is suitably carried out in a fluidised bed reactor, asdescribed in the aforesaid British Patent Specification No. 1,387,335.The esterification, polyesterification and polycondensation reactionsare carried out simultaneously in the bed, for example at a bedtemperature within the range 160° to 240° C. so that polymer, esters andterephthalic acid are present at intermediate stages of the process.However, it is difficult to avoid some aggregation of solid particlesoccurring and we have now found that agglomeration can be reduced if theprocess is carried out using the modified fluidised bed according to ourinvention.

The esterification stage is primarily carried out in the inner coolerzone or zones of the bed, for example starting at a temperature aboutthe melting point of the initial ester product (120° C.) and typicallyincreasing from 120° C. to 180° C. The temperature of the cooler zone(s)is conveniently maintained by controlling the temperature of the jet orjets introducing a fluidising gas mixture of ethylene oxide and an inertgas (for example nitrogen). The polycondensation and polyesterificationmay start in the cooler zone(s) and stop when the product melting pointreaches the temperature of the cooler zone(s). The poly-condensation andpolyesterification reactions may then continue under thermal control asthe particles are transferred from the cooler zone(s) to the hotterzone(s) surrounding said cooler zones. The temperature of the hotterzone(s) is maintained suitably within the range 180° to 210° C.,conveniently by controlling the temperature of the fluidising gas (forexample an inert gas, such as nitrogen).

It will be appreciated that a temperature gradient may exist both withinthe cooler zone(s) and within the hotter zone(s) and between said zones;that is, it is unlikely that there is a precise temperature boundarybetween the zones. Nevertheless, there is an overall difference in thetemperature ranges associated with the hotter and cooler zones asmentioned above.

It will also be appreciated that as the process proceeds, the averagemelting point of the product increases. Accordingly, the process may becontrolled by reducing the temperature difference between the coolerzone(s) and the hotter zone(s), conveniently by increasing thetemperature of the cooler zone(s), for example up to a temperature equalto that of the hotter zone(s), and finally to a temperature which isgreater than that of the initially hotter zone(s), typically within therange 210° to 260° C. It will be appreciated that in the final stages ofthe process, wherein further polycondensation and/or polyesterificationreactions are taking place, the outer cooler zone(s) surround the innerhotter zone(s) in the manner described previously for the furtherpolymerisation of low molecular weight poly(ethylene terephthalate).

In order that the invention may be clearly understood it will bedescribed by the following Examples with reference to the accompanyingdrawings in which

FIG. 1 is a cross-sectional elevation of an apparatus according to theinvention,

FIG. 2 is a digrammatic view of a perforated fluidising bed plate,

FIG. 3 is a diagrammatic view of a jet in the form of a disperser,

FIG. 4 is a diagrammatic view of a jet in the form of a hollow shaftedstirrer provided with holes, which is suitable for use as a combined jetand stirrer,

FIG. 5 is a cross-sectional elevation of an apparatus according to theinvention incorporating the disperser shown in FIG. 3.

Referring to the apparatus shown in FIG. 1, circular steel plate 1 has acentral cavity 2 which communicates with gas feed-pipe 3. A similarlyshaped circular plate 4 has a central cavity 5 which communicates with asecond gas feed-pipe 6 and also with pipe 7 which leads to amicromanometer (not shown). Plate 4 is secured to plate 1 by a series ofset-screws 8 which pass through holes in plate 4 and screw into tappedholes in plate 1. Externally threaded jet member 9 is screwed into atapped central hole in plate 4 and is supported thereby. Stainless steelsintered disc 10 rests on the upper edge of plate 4 and is provided witha central hole through which the upper end of jet member 9 protrudes.Alternatively, sintered disc 10 may be replaced by a perforated nickeldisc 10' as shown in FIG. 2. Sintered disc 10 is held in place byrecessed collar 11 which is secured to the edge of plate 4 by a seriesof set-screws 12 which pass through countersunk holes 13 in collar 11and screw into tapped holes in the edge of plate 4. Thermocouple 14passes through a hole in the wall of collar 11. A thick-walled glasstube 15, provided with a protruding edge 16 at its lower extremity, issupported by collar 11. A gas-tight seal between collar 11 and glasstube 15 is provided by P.T.F.E. gasket 17. Glass tube 15 is clamped inplace by encircling flat ring 18 which interacts with protruding edge 16via gasket 19. Ring 18 is provided with holes to accept the upper endsof threaded rods 20, the lower ends of which screw into tapped holes inthe edge of plate 1. The upper ends of rods 20 are provided with springs21 which are compressed by nuts 22 via washers 23 (Nuts 22 are onlyscrewed down finger tight so that the resilience of springs 21 mayobviate undue pressure on edge 16 of tube 15). The end of feed-pipe 6remote from the apparatus leads to a coiled tube located in an oven (notshown). Pipe 3 is provided with a gas-tight T-piece 24 in which islocated thermocouple 25. The end of pipe 3 remote from the apparatusleads to a separate coiled tube located in a second oven (not shown).The sintered disc used was one supplied by Accumatic EngineeringLimited, under the Registered Trade Mark "Sintercon", fine grade 3. Ithad a thickness of 0.100", maximum particle retention of 0.0008-0.0010"and permeability of 7.5×10⁻⁸ cm². The perforated nickel disc 10' wascomprised of four circular rows of holes 26 having 52, 39, 26 and 13holes respectively, each hole having a diameter of 0.5 mm.

The diameter of the jet 9 was 4 mm and the diameter of the sintered disc10 or perforated disc 10' was 72 mm.

In use, the particulate material is charged to the vessel defined by thewall of glass tube 15 and sintered disc 10, which acts as agas-permeable baseplate. Fluidising gas is then supplied throughfeed-pipe 6, having been heated to the desired temperature by passagethrough the aforementioned heating coils. The secondary gas to jet 9 issupplied via feed-pipe 3, having been heated by passage through itsheating coils. The working temperatures of the two gas supplies aremeasured by thermocouples 14 and 25, respectively, the pressure offluidising gas being indicated by the micromanometer. In order to keepthe apparatus up to temperature, the whole assembly was enclosed in anoven which was maintained at the temperature of the fluidising gas.

The jet member 9 may be replaced by an externally threaded disperser asshown in FIGS. 3 and 5. The disperser (as shown in FIG. 3) comprises ahead 27 carrying a plurality of side ports 28 (for example, 8 holes eachof 3 mm diameter) and an externally threaded portion 29. The disperseris screwed into the tapped central hole in plate 4 (FIG. 5) and issupported thereby. The gas to the disperser is supplied by feed-pipe 3as described above. The disperser is provided with a spigot 30 forattachment to a stirrer (not shown) rotatable by a motor (not shown).

The jet member 9 may be replaced by a combined stirrer and jet member(as shown in FIG. 4), comprising a hollow stirrer shaft 31 carryingstirrer blades 32, said shaft 31 being provided with a plurality ofholes 33 (for example, 4 to 8 holes, each of 0.5 mm diameter). The upperportion of stirrer shaft 31 is attached to and rotated by a motor (notshown) and the lower portion of said shaft rotates in a central bearing(not shown) in plate 4. The gas is fed to the lower portion of the shaft(and thereby to the holes) via the feed-pipe 3 as described above.

Polymerisation of low M wt Poly(ethylene terephthalate)

General Procedure

The polymerisations were carried out on poly(ethylene terephthalate)pre-polymer produced by the above-mentioned melt phase process. It hadan IV of 0.38 and was shown to be 50% crystalline. The material wasground and sieved to yield fractions having particles either havingdiameters >300 microns or between 150 and 300 microns.

In each experiment, the apparatus was switched on and heated gas passedthrough both sintered disc (10) and jet (9) until the jet and fluidising(bed) gas streams stabilised at the desired temperature. The charge ofpre-polymer (250 g) was then introduced into glass vessel (15) and thegas flows adjusted to the desired levels. This was taken as zero time.Jet and bed temperatures were carefully monitored and maintained attheir pre-determined values and small samples of pre-polymer werewithdrawn at 30 minute intervals and analysed for IV. The reading on themicromanometer was also monitored, as a fall in pressure indicated theonset of agglomeration or sintering of the particles.

EXAMPLE 1 and 2 Using the above general procedure, experiments werecarried out using jet temperatures in the range 235° to 242° C. and bedtemperatures in the range 206° to 224° C., the procedure being continueduntil sintering occurred or until a reasonable increase in IV had beenachieved. The pre-polymer had a particle size from 150 to 300μ inExample 1 and >300μ in Example 2. The results are given in Table 1below.

                  TABLE 1                                                         ______________________________________                                        Ex   Gas Flow (liters/min)                                                                        Temp (° C.)                                                                      Sinter                                          No   Jet      Bed       Jet  Bed  Time (min)                                                                             IV                                 ______________________________________                                        1    12       25        236  206  NS   120   0.57                             2    12       12        235  206  NS   290   0.75                             C1   12       12        236  213       125   0.71                             C2   15       30        242  224        5    NM                               ______________________________________                                         NS = not sintered                                                             NM = not measured                                                        

It will be seen from the above results that the bed temperature ispreferably kept below 210° C. and that smaller particle sizes reduce thepolymerisation time. Comparative examples C1 and C2 show the effect ofincrease of bed temperature. When the procedure was repeated with asimple fluidised bed, it bubbled freely at 200° C. but when thetemperature was raised, the bed sintered at 225° C., a temperature wellbelow that of the jet in Examples 1 and 2 above.

EXAMPLES 3-7

The general procedure was again followed, but this time polymerisationwas continued until an IV of 0.67 was reached without sintering. In eachcase the bed temperature was maintained between 205° and 210° C., thejet temperatures ranging from 235° to 265° C. The results are given inTable 2.

                  TABLE 2                                                         ______________________________________                                             Gas Flow                                                                 Ex   (liters/min)                                                                              Jet Temp  Time    Particle Size                              No   Jet     Bed     (° C.)                                                                         (mins)  (μ)                                   ______________________________________                                        3    12.0    12.0    235     240     150/300                                  4    4.3     4.3     246     240     150/300                                  5    5.9     4.3     248     210     150/300                                  6    8.0     4.3     247     330     > 300                                    7    8.0     4.3     265     330     > 300                                    ______________________________________                                    

It will be seen from the above results that the jet temperature can beraised considerably without sintering occurring, provided that the bedtemperature is kept below 210° C. Again, larger particle sizes increasepolymerisation time.

Polymerisation of Terephthalic Acid

General Procedure

The polymerisations were carried out on terephthalic acid, particle size50 to 200 microns, containing 0.175% by weight titaniumdicyclopentadienyl dichloride as a polyesterification catalyst and 0.05%by weight antimony trioxide as a polycondensation catalyst together with0.5% titanium dioxide as opacifier.

In each experiment, a heated nitrogen steam was passed through adisperser (with 8 holes, each of 3 mm diameter) with an attached stirrerrotating at 60-120 revs/min, (Example 8) or through a rotating hollowshafted stirrer (60-120 revs/min) provided with 4 holes each of 0.5 mmdiameter in its shaft (Examples 9 and 10), and a heated mixture oftriphenyl phosphine (3 mg/1) and nitrogen was passed through aperforated nickel disc 10' until the jet and fluidising (bed) gasstreams stabilised at the desired temperatures with the charge ofterephthalic acid (400 g) present in the glass vessel 15 and the gasflows adjusted to the desired levels. Zero time was taken when the jetand bed temperatures stabilised, at which time the jet gas was chargedto a mixture of ethylene oxide (15% by volume) and nitrogen (85% byvolume). Jet and bed temperatures were carefully monitored and graduallyincreased throughout the polymerisation.

EXAMPLES 8-10

Using the above general procedure, experiments were carried out usingjet temperatures in the range 122° to 252° C. and bed temperatures inthe range 180° to 210° C. The jet temperatures were maintained for 1-2hours at the lowest temperature, and then increased linearly over 4-6hours from the lowest temperature to 180° C., then maintained at180°-210° C. for about 2 hours, and finally increased over 2-4 hours upto 230°-250° C. The results are shown in Table 3.

                  TABLE 3                                                         ______________________________________                                        Ex  Gas Flow (liters/min)                                                                        Temp (° C.)                                                                          Time %                                       No  Jet      Bed       Jet    Bed    (hrs)                                                                              Reaction                            ______________________________________                                          8 10       8         146-249                                                                              180-205                                                                              13   89                                   9  10       8         130-252                                                                              185-210                                                                              8    95                                  10  10       8         122-230                                                                              186-206                                                                              9    93                                  ______________________________________                                    

The percentage reaction was determined by measuring the residue aftervolatilising the terephthalic acid in a thermogravimetric apparatus(TGA).

What we claim is:
 1. A process for causing a material to undergo one ormore thermally-initiated chemical reactions or one or more physicalchanges in which the material or at least a component thereof is in asolid particulate state and is maintained as a fluidised bed, thefluidising gas being supplied in two or more zones comprising at leastone cooler zone in which the temperature of the gas is below atemperature at which said particulate material is susceptible to anundesirable thermal transformation and at least one hotter zone in whichthe temperature of the gas is above the temperature at which saidparticulate material is susceptible to the undesirable thermaltransformation, the zones being arranged so that the particles of thebed are constantly moving between them, the dwell time in the hotterzone or zones being sufficiently short so that the undesirabletransformation does not take place before the particles move into thecooler zone or zones, the temperature of the said zones being selectedso as to cause the chemical reaction(s) or physical change(s) to takeplace.
 2. A process as claimed in claim 1 wherein the temperature in atleast the hotter zone(s) is sufficiently high to cause the chemicalreaction(s) or physical change(s) to take place.
 3. A process as claimedin claim 2 wherein the cooler fluidising zone(s) is arranged to surroundthe hotter fluidising zone(s) so that at least some of the particles areborne upwards in the hotter gas, and on reaching the top of the bed,spill over into the cooler surrounding gas, where they travel to thebottom of the bed and are later re-entrained in the hotter gas.
 4. Aprocess as claimed in claim 3 wherein the bed comprises either a singlehotter zone at the centre of the bed, or a plurality of hotter zonesspaced about the area of the bed sufficiently well separated to providea comparatively large reservoir of cooler gas around them.
 5. A processas claimed in claim 4 wherein the cross-sectional ara of the hotterzone(s) will be smaller than that of the surrounding cooler zone(s), sothat the hotter particles are concentrated in a narrower zone or zones.6. A process as claimed in any one of the preceding claims wherein thesolid particulate material is heated by controlling the relativetemperatures of the streams of gas to the hotter and cooler zones.
 7. Aprocess as claimed in claim 6 wherein the streams of gas are provided byseparate gas supplies which are heated independently.
 8. A process asclaimed in any one of the preceding claims wherein the bed material is alow molecular weight poly(ethylene terephthalate) "pre-polymer" (ashereinbefore defined) and wherein said pre-polymer is furtherpolymerised by heating.
 9. A process as claimed in claim 8 wherein thefluidising gas in the cooler zone(s) is maintained between 205° and 210°C. and the fluidising gas in the hotter zone(s) is maintained between210° and 265° C.